2. An aqueous solution containing 0.1 g/L pyruvic acid (CH3COCOOH) is separated

Question

2. An aqueous solution containing 0.1 g/L pyruvic acid (CH3COCOOH) is separated from pure water by a membrane permeable to water, all at 25 degree C. i. Calculate the osmotic pressure that you expect for this solution. Assume ideal behavior of the molecular solute (no dissociation). ii. The observed osmotic pressure is 4.86 x 10^-2 atm. Use this value to estimate the pKa for the acid dissociation of pyruvic acid at 25 degree C. iii. What is i, the van?t Hoff factor? Does its value make sense for a weak acid?

Explanation / Answer

pyruvic acid molar mass = 88 g /mol

concentration = 0.1 g / L

molarity = 0.1 / 88 mol /L

             =1.136 x 10^-3 M

T = 25 + 273 = 298 K

osmotic pressure = P = M R T

P = 1.136 x10^-3 x 0.0821 x 298

P = 0.0278 atm

osmotic pressure = 0.0278 atm

(ii) observed osmotic pressure = 4.82 x 10^-2 atm

    4.82 x 10^-2   = M x 0.0821 x 298

M = 1.97 x 10^-3

Ka = (1.136 x10^-3)(1.136 x10^-3)/(1.97 x 10^-3 )

Ka = 6.55 x10^-4

pKa = -log Ka = -log (6.55 x10^-4)

pKa = 3.18

(iii) vanthoff factor (i) = observed O P / calculate O P

                                    = (4.86 x 10^-3) / 0.0278

                                    = 0.175

it is reasonable for weak acid . i value < 1

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